1. Technical Field
This invention relates to cordless telephones and more particularly to an apparatus and method employed in a cordless telephone for automatically modifying a frequency hopping sequence in the presence of interference.
2. Description of the Prior Art
Recent rulings promulgated by the Federal Communication Commission (FCC) in the utilization of spread spectrum systems, including a frequency hopping system, now allow for increased spectral utilization for cordless telephones. Frequency hopping systems spread their energy by changing, or hopping the center frequency of the transmission many times a second in accordance with a pseudo randomly generated list of communication channels. The result is a significantly higher signal to noise ratio than may be achieved by conventional techniques such as amplitude modulation that uses no bandwidth spreading.
These recent FCC rulings provide general guidance for how cordless telephones may operate in a spread spectrum system. They do not, however, provide specific guidance as to standards required for the compatible operation of these cordless telephones. Because such standards have not yet been adopted for determining the configuration of cordless telephones intended for spread spectrum operation, there is a possibility of interference between cordless telephones operating in different spread spectrum configurations within a frequency band designated for such operation. By way of example, two frequency hopping telephones may be configured to frequency hop at different rates. In fact, in view of the guidance presently available covering the configuration of these telephones, one telephone may hop at twice the rate of the other. Thus, if two frequency hopping cordless telephones that are hopping at different rates and operating within reception range of each other happen to appear on the same channel at the same time, there is a likelihood of interference occurring between these two telephones. This interference could minimally cause a distortion of information and, at worst, a loss of synchronization between a handset or remote unit and an associated base unit of one and possibly both of these cordless telephones. This interference could continue as both cordless telephones continue to hop to and appear on this channel and possibly other common channels at the same time.
Another source of interference for a cordless telephone operating in a frequency hopping system is noise or other signal appearing continually on the channel hopped to by the cordless telephone. This type of interference will continue to interfere with the cordless telephone each time it hops to this channel.
As cordless telephones configured for frequency hopping operation increase in use, the likelihood of interference for such a telephone due to other telephones within its reception range being on the same channel at the same time will increase. And the interference from noise or other signal appearing on a channel in use by the frequency hopping telephone also will have an adverse effect on the operation of these telephones. It is therefore desirable to minimize the interference encountered by a cordless telephone from other cordless telephones and also noise and other signals while operating in a frequency hopping system.